cros: Fix WebviewLoginTest.Basic multiple login attempts.

ShowSigninScreenForCreds is used to automatically fill-in sign-in
forms and submit it. WebviewLoginTest.Basic does that manually
thus should not call it. Otherwise, /_/embedded/signin/challenge
is called multiple times with wrong email (empty) and could trigger
FakeGaia failures.

BUG=472095

Review URL: https://codereview.chromium.org/1069423004

Cr-Commit-Position: refs/heads/master@{#324353}

chrome/browser/chromeos/login/webview_login_browsertest.cc

@@ -46,8 +46,6 @@ class WebviewLoginTest : public OobeBaseTest {
           "});"));
 
     content::DOMMessageQueue message_queue;
-    GetLoginDisplay()->ShowSigninScreenForCreds("", "");
-
     std::string message;
     ASSERT_TRUE(message_queue.WaitForMessage(&message));
     EXPECT_EQ("\"GaiaReady\"", message);